The present invention relates to a silencing device for attenuating exhaust noise in an exhaust system of an internal combustion engine and, more particularly, to a silencing device suitable for use with an exhaust system of the type having a double-layer exhaust pipe.
Various kinds of silencing devices are used with automotive vehicles in order to attenuate noise generated by exhaust gases which are emitted from internal combustion engines. The silencing devices known in the art may generally be classified into three types, i.e., an absorption type device, an expansion type device, and a resonance type device. The absorption type device is constructed to absorb energy of sound by use of glass wool or like sound absorbing material. The expansion type device includes an expansion chamber which is defined in a part of an exhaust pipe, so sound waves may be reflected by the wall of the chamber to cancel each other by interference. The resonance type device, which is believed to be highest in silencing performance, includes a resonance chamber which is configured to produce waves whose phases are opposite to those of incoming waves of exhaust noise. Ordinary exhaust silencing devices, or mufflers, are implemented with the combination of such three different types of devices so as to make the most of their inherent effects.
As regards the resonance type scheme, noise of low frequencies cannot be attenuated unless the resonance chamber is provide with a substantial length. However, in the case of an automotive vehicle, the space available below the frame is too limited to provide a resonance chamber in a muffler. In an ordinary muffler, therefore, it is extremely difficult for low frequency noise to be effectively attenuated.
An exhaust pipe of an automotive vehicle is often comprised of concentric inner and outer conduits so as to reduce radiation of noise and, at the same time, to thermally insulate the interior of the pipe from the outside. In such a double-layer exhaust pipe, an air gap is defined between the inner and outer conduits over a substantial length. The long air gap may be used as a resonance chamber, as disclosed in U.S. Pat. Nos. 3,780,826, 3,648,803, and 3,338,331 by way of example.
In all of the prior disclosures mentioned above, the inner conduit is formed with numerous perforations to provide communication between the inside and the outside of the inner conduit. This suffers from various drawbacks, however. Specifically, forming the perforations through the inner conduit requires extra machining steps. Moreover, because the diameter, number, positions and others of the perforations have critical influence on the resonance-based silencing effect, greatest possible care should be given to the machining. Especially, while the amount of attenuation and, therefore, the silencing effect becomes greater with the increase in the total area of the perforations, forming large openings through the wall of the inner conduit is considerably difficult. Another problem is that the perforations constitute resistance to exhaust gases which flow through the inner conduit.